Bolts from the blue

"I recently witnessed the most spectacular and unexpected natural event I have seen," wrote commercial pilot John Hammerstrom of Tavernier, Florida, in 1993. When he was flying over Central America, Hammerstrom had glimpsed strange jets of blue light springing up from a thundercloud. Such jets, and the giant flashes dubbed "sprites" that dance in the upper atmosphere above thunderstorms, have been baffling atmospheric researchers since their first reliable sightings in 1989. And this summer, a host of new projects are under way in the US to try to understand what processes are at work.

While it's clear that they are intimately associated with thunderstorms, sprites are not lightning as we understand it. They appear as giant nebulous glows that occur up to about 90 kilometres above the Earth. Their shapes are complex and diverse; observers have described them as carrot and turnip-shaped, or "jellyfish". The light is usually red, sometimes ...

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